SEM Imaging - Magnification and Deflection System - Focussing (contamination dots) - Imaging

means to be good in SEM imaging. Motivation To be good in e-beam lithography means to be good in SEM imaging.

Tip Geometry & Crossover a) LaB6 gun b) thermal field emission (Gersley, J. Appl. Phys. 65 (3), 914 (1989))

Filament Types source type brightness [A/cm²/sr] source size energy spread [eV] vacuum [torr] tungsten thermionic ~10 5 25µm 2 - 3 10 6 LaB 10µm 8 thermal FE (Schottky) 20nm 0.9 9 cold FE 5nm 0.22 SPIE HANDBOOK OF MICROLITHOGRAPHY, MICROMACHINING AND MICROFABRICATION Volume 1: Microlithography, Chapter 2.2

Beam deflection (E-static/magnetic) Either magnetic or electrostatic fields can be used to focus electrons just as glass lenses are used to focus rays of light. Electro-static Electro-magnetic F = q · (E + v  B) EM lenses more simple Fast deflection

Beam deflection (E-static/magnetic) Electron lenses have very poor performance (spherical and chromatic aberrations) compared to light lenses; thus electrons must be kept very close to the axis - small  Spherical aberration Chromatic aberration

Beam Deflection E-static or B? Magnetic deflection - lower aberrations (as with electrostatic lenses (= condensor in gun)) - but max. frequency is limited by the coil resonances to ~10 MHz. - max. angle limited by off-axis aberrations to about 1 degree - Trade off between resolution and field size on-axis aberrations increase with increasing working distance deflection angle and thus off-axis aberrations decrease with increasing working distance   Compromise field size is much smaller than most chips!

Beam Blanker Beam blanker off Beam blanker on Filament Anode +250V GND Beam blanker Aperture

Focus / Stigmation Correction

Focussing - Contamination Dots top view side view (Images taken during the acceptance test of Raith 200 at the University of Neuchâtel (Switzerland) 4/98)

Focussing = WD and Stigmation (Image taken at KTH Stockholm (Sweden), see http://www.nanophys.kth.se/nanophys/facilities/nfl/sem-ebeam.htm)

Influence of Acceleration Voltage Low (short penetration depth) High (large penetration depth) + Clear surface structures + Less damage + Less charge up + Less edge effect – Lower resolution + Higher resolution – Unclear surface structures – More edge effects – More sample damage (heating) (A guide to Scanning Microscope Observation, Jeol web page 1999)

Influence of Beam Current-Aperture Low (small aperture) High (large aperture) + Higher resolution + Less damage (heating) + Larger depth of focus – Grainy image + Smooth image + Good signal-to-noise – Deteriorated resolution – More damage (heating) – Lower resolution – Smaller depth of focus (A guide to Scanning Microscope Observation, Jeol web page 1999)

Influence of Working Distance Small Large + Higher resolution – Smaller depth of focus + Larger depth of focus – Lower resolution (A guide to Scanning Microscope Observation, Jeol web page 1999)

Important rule for SEM imaging Take always the low magnification images first!!!